This invention relates to a process for continuous polymerization of a styrene monomer using a tubular reactor having fixedly set therein a plurality of mixing elements having no moving parts to produce a styrene resin having a high molecular weight and a narrow molecular weight distribution.
Industrial production of styrene resins has generally been carried out by a batchwise suspension polymerization method, and a continuous bulk polymerization method using a tower-type or tank-type reactor.
The batchwise suspension polymerization can give polystyrene having a weight average molecular weight (simply molecular weight hereinafter) of as high as 350,000 to 450,000, and its properties have been highly evaluated on the market. The advantage of this polymerization method is that since the monomer is polymerized while it is dispersed in water, the viscosity of the polymerization solution increases little as the polymerization proceeds. However, it involves a high cost because a large amount of a dispersing agent is used and the waste water should be treated. Moreover, the clarity of the polymer obtained is not so good. These problems are difficult to solve.
The continuous bulk polymerization method has excellent economy and productivity. However, it requires a large-sized plant, and in view of its process, there is a limit to mixing of a high-viscosity polymer solution at a stage where the polymerization has progressed, and to the removal of the heat of the reaction. It is extremely difficult to produce polystyrene having a molecular weight of more than 300,000 by this method. Approaches, from the standpoint of both the reaction apparatus and conditions, have previously been made to the solution of this problem in the continuous bulk polymerization. For example, there have been proposed a method in which the area of heat transmission is increased by using a draft tube in a multistage stirred tank type reactor provided with stirring vanes of a special structure (Japanese Patent Publication No. 610/1972) and a method in which 20 to 70% by weight of a styrene monomer is polymerized in the presence of a specific organic peroxide in a first stage reactor, and the polymerization is carried out at a higher temperature in a final reactor (Japanese Laid-Open Patent Publication No. 173107/1983).
However, in the multistage stirred tank-type reactor, the residence time of the polymer solution varies greatly, and a deadspace is liable to form. Thus, the reactor has a low volume efficiency, and the quality of the polymer tends to be degraded owing to the residence of the polymer. Furthermore in a commercial scale plant, the efficiency of mixing the highly viscous polymer solution of a high viscosity in a stage where the polymerization has proceeded to a great extent decreases, and the removal of the heat is difficult. Furthermore, because the polymer solution is of high viscosity, a great stirring power is required, and the energy cost becomes high. Accordingly, it is necessary to reduce the conversion in the final reactor or to add a solvent. Consequently, the molecular weight of the polymer is low, and the volume efficiency and productivity are reduced.
In an attempt to solve this problem, there has been proposed a method in which a tubular reactor having fixedly set therein a plurality of mixing elements having no moving parts is used in a later polymerization stage of a stirred tank-type reactor, and the polymerization is further carried out in it at an elevated temperature thereby to increase the conversion. However, since the reaction temperature is higher, the molecular weight distribution of the resulting polymer is broadened, and its mechanical properties are not satisfactory.
The present inventors noted, and studied, the use of tubular reactors, linked to each other, having fixedly set therein a plurality of mixing elements having no moving parts in which the polymer solution flows as a plug flow having little variation in residence time and can be mixed even when its viscosity is relatively high. A continuous bulk polymerization method for polystyrene using tubular reactors of this type connected to each other is shown, for example, in Japanese Laid-Open Patent Publication No. 1515/1984. According to this method, the problems of mixing and heat removal are solved by setting up a circulating line in the initial stage of the polymerization and circulating the polymerization solution through the line. Furthermore, this leads to a reduction in energy cost as compared with the case of using the multistage stirred tank-type reactor and also a reduction in the variations of the residence time of the polymer solution in the reactor.
Polystyrene obtained by this method, however, has a molecular weight of about 350,000 (determined on a sample taken at the exit of the reactor). If attempts are made to obtain polystyrene of a higher molecular weight, the rise of the viscosity of the polymer solution causes a large pressure drop in the tubular reactor. As a result, the energy cost rises and the plant cannot be operated stably. If the temperature in the latter half of the polymerization is sufficiently elevated in this method, the pressure drop in the tubular reactor can be reduced. But the molecular weight of the polymer decreases to, for example, about 250,000.
In this method, the latter half of the polymerization may be carried out in the presence of a peroxide, and this can increase the conversion, shorten the length of the polymerization line, and reduce the pressure drop. But the molecular weight of the polymer is decreased. In view of the foregoing background, the present inventors made extensive investigations on a process for producing high-molecular-weight styrene resins by continuous bulk polymerization using tubular reactor, linked to each other, having fixedly set therein a plurality of mixing elements having no moving parts. It has consequently been found that when a polymerization solution containing a monomer, an organic solvent and a specific organic peroxide is polymerized in the early stage while it is being circulated within a circulating line, and a minor part or a major part of the initial-stage polymerization solution is introduced into a polymerization line following the circulating line and polymerized, a styrene resin having a high molecular weight and a narrow molecular weight distribution is obtained, and the pressure drop in the polymerization line is greatly reduced, in spite of using the organic solvent and organic peroxide which are known to decrease molecular weight when used in a polymerization process involving the aforesaid multistage stirred tank-type reactor.